The present invention relates generally to hydraulically driven piston and barrel assemblies, and more particularly hydraulically-actuated fuel injectors utilizing stepped piston and barrel assemblies.
Hydraulically driven piston and barrel assemblies are utilized in diverse ways in a wide variety of related and unrelated machines. In most of these applications, the piston reciprocates in a piston bore defined by the barrel between a retracted position and an advanced position. The piston is driven from its retracted position toward its advanced position by a hydraulic pressure force produced by a pressurized fluid acting on one end of the piston. In some instances, it is desirable to control the initial movement rate of the piston. For example, piston and barrel assemblies are utilized in hydraulically-actuated fuel injectors to pressurize fuel within the injector for each injection event. Over time, engineers have discovered that the injection rate profile can be controlled by controlling the movement rate of the piston. Controlling the initial injection rate is especially important because of the strong influence that initial injection rate shape has on the quality of emissions leaving a particular engine.
Known hydraulically-actuated fuel injection systems and/or components are shown, for example, in U.S. Pat. No. 5,121,730 issued to Ausman et al. on Jun. 16, 1992; U.S. Pat. No. 5,271,371 issued to Meints et al. on Dec. 21, 1993; and, U.S. Pat. No. 5,297,523 issued to Hafner et al. on Mar. 29, 1994. In these hydraulically-actuated fuel injectors, a spring biased needle check opens to commence fuel injection when pressure is raised by an intensifier piston/plunger assembly to a valve opening pressure. The intensifier piston is acted upon by a relatively high pressure actuation fluid, such as engine lubricating oil, when an actuator driven actuation fluid control valve opens the injector""s high pressure inlet. In these hydraulically actuated fuel injectors, the actuator comprises a solenoid. Injection is ended by deactivating the solenoid to release pressure above the intensifier piston. This in turn causes a drop in fuel pressure causing the needle check to close under the action of its return spring and end injection.
While these hydraulically-actuated fuel injectors have performed magnificently over many years, there remains room for improvement, especially in the area of shaping an injection rate trace from beginning to end to precisely suit a set of engine operating conditions.
Over the years, engineers have discovered that engine emissions can be significantly reduced at certain operating conditions by providing a particular injection rate trace. In many cases, emissions are improved when the initial injection rate is controllable, and when there is a nearly vertical abrupt end to injection. While these prior hydraulically-actuated fuel injection systems have some ability to control the injection rate shape, there remains room to improve the ability to control the injection rate shape with hydraulically-actuated fuel injection systems.
The invention is directed to overcoming one or more of the problems set forth above.
A hydraulically actuated fuel injector has an injector body that includes a barrel and defines a nozzle chamber, a needle control chamber and a nozzle outlet that opens to the nozzle chamber. The injector body further includes an actuation fluid inlet and an actuation fluid drain. A barrel defines an actuation fluid cavity and a piston bore, which includes an upper bore and a lower bore. Hydraulic means are included within the injector body for pressurizing fuel in the nozzle chamber. The hydraulic means includes a piston with a stepped top slidably received in the piston bore and moveable between a retracted position and an advanced position. The stepped top of the piston includes a first area that is separate from a second area. The first area and the upper bore define an upper cavity connected to the actuation fluid cavity through a relatively unrestricted flow area when the piston is in the retracted position. The second area and the lower bore define a lower cavity connected to the actuation fluid cavity through a relatively restricted flow area when the piston is in the retracted position. The first area is exposed to fluid pressure in the upper cavity and the second area is exposed to fluid pressure in the lower cavity over a portion of the piston""s movement from the retracted position toward the advanced position. A needle valve member is positioned in the nozzle chamber and is moveable between an open position in which the nozzle outlet is open and a closed position in which the nozzle outlet is blocked. The needle valve member includes a closing hydraulic surface exposed to pressure in the needle control chamber. A needle control valve includes an actuator and is attached to the injector body and is moveable between an off position in which the needle control chamber is opened to a source of high pressure fluid and an on position in which the needle control chamber is opened to a low pressure passage. An actuation fluid control valve includes the actuator and is moveable between a first position in which the actuation fluid inlet is open to the actuation fluid cavity and a second position in which the actuation fluid inlet is closed to the actuation fluid cavity.